Method for treating a molten salt with water vapor

ABSTRACT

This invention relates to the strengthening of alkali metal silicate glass articles through an ion exchange reaction employing a bath of molten alkali metal salt as the source of exchanging ions. More specifically, this invention relates to a means for typing up calcium ions present in the bath of molten salt, which deleteriously affect the strengthening capability of the bath, by passing water vapor through the molten salt.

United States Patent Bartholomew 1 51 May 9,1972

[54] METHOD FOR TREATING A MOLTEN SALT WITH WATER VAPOR [72] lnventor:

[73] Assignee: Corning Glass Works, Corning, NY.

[22] Filed: Nov. 19, 1969 [21] Appl. No.: 878,225

Roger F. Bartholomew, Corning, NY.

52 u.s.c1 ..65/30,65/32,65/l34,

65/99, 65/182, 65/1 16 51 mm. ..C03c l5/00,C03b18/O0 5s FieldoiSearch..65/30,ll6,32,l34,99,l82

[56] References Cited UNITED STATES PATENTS 5/1967 Ray ..65/3O 8/1968Lewek ..65/30 3,477,834 11/1969 Morris 1.65/30 3,556,757 l/l97lKozlowski et al 65/30 X 2,263,489 11/1941 Day ..65/1 16 3,259,517 7/1966Atwell... 65/30 X 3,375,155 3/1968 Adams ...65/30 X 3,529,946 9/1970Fischer et al ..65/1 16 X Primary Examiner-Frank W. MigaAttorney-Clarence R. Patty, Jr. and Clinton S.Janes,.1r.

[57] ABSTRACT 5 Claims, No Drawings METHOD FOR TREATING A MOLTEN SALTWITH WATER VAPOR In the past, baths of molten salts have been utilizedin treating glass articles for various purposes such as staining,surface coating, thermal tempering, and, more recently, chemicalstrengthening. This latter process contemplates two fundamentallydifferent techniques but both techniques rely upon the same basicmechanism, viz., the exchange of ions occurring between a molten saltand the surface of the glass article. The instant invention was designedto alleviate certain problems which had been encountered in thesechemical strengthening techniques but, while having special utility forthese purposes, should not be so restricted.

The first method of ion exchange strengthening, exemplified by U.S. Pat.No. 2,779,136, comprises exposing a sodium and/or potassium silicateglass article to a bath of a molten lithium salt operating at atemperature above the strain point of the glass for a sufficient lengthof time to cause the replacement of sodium and/or potassium ions withlithium ions in the surface of the article. Inasmuch as this exchange isundertaken at temperatures above the strain point of the glass, viscousflow and molecular rearrangement can occur in the surface of the glassso, in reality, a new glass composition is formed in the surface layer.This lithium-containing glass has a lower coefficient of thermalexpansion than the parent sodium and/or potassium glass such that whenthe article is cooled to room temperature a surface compression layer isdeveloped within the surface of the article. The so-developedcompression layer imparts greatly enhanced mechanical strength to thearticle.

In the second method of ion exchange strengthening, typified by BritishPat. No. 917,388, comprises exposing an alkali metal silicate glassarticle to a bath of molten salt of a monovalent metal having a largerionic diameter than that of the alkali metal present in the glass,operating at a temperature below the strain point of the glass, for asufficient length of time to cause replacement of the alkali metal ionsin the surface of the glass with the larger monovalent metal ions. Sincethis exchange is carried out at temperatures below the strain point ofthe glass, viscous flow therein cannot take place such that the largermonovalent metal ions are crowded into sites within a surface layer ofthe glass previously occupied by the small alkali metal ions. Thiscrowding of the larger ions into the small ion sites sets up compressivestresses in a surface layer within the article which, in turn, increasethe mechanical strength of the article.

British Pat. No. 966,733 relates to a specific improvement upon saidBritish Pat. No. 917,388, viz., the unexpected utility of alkali metalaluminosilicate glasses in this ion exchange strengthening practice.Hence, laboratory experimentation had demonstrated that, although manyglass compositions could be initially strengthened by thislarge-ion-for-small-ion type exchange, the improvement in strengthimparted to such articles was essentially lost after minor abrasion ofthe surface thereof. And, inasmuch as substantially all applications forglass products contemplate some measure of surface abuse, even if onlythat commonly encountered in normal handling and shipping, theenhancement in strength provided by the ion exchange treatment was tooephemeral to be useful.

However, as is pointed out in British Pat. No. 966,733, glassesconsisting essentially, by weight on the oxide basis, of about -25percent alkali metal oxide, 5-25 percent A1 0 and the remainder SiOexhibit a great improvement in strength when exposed to thelarge-ion-for-small-ion type exchange and this increase is essentiallymaintained even after substantial surface abrasion. Therefore, thealkali metal aluminosilicate glasses are stated to demonstrate excellentabraded strength, Le, a permanent or practical strength, whereas otherglass compositions, e.g., the common sodalime-silica glasses ofcommerce, do not. This situation has resulted in all the presentlymarketed glass articles which have been ion exchange strengthened beingmade from compositions in that field.

The presence of impurities in the bath of molten salt can result incrazing, etching, and other kinds of surface attack on the glass or cancause the bath to become non-strengthening, i.e., a surface compressionlayer is not formed on the glass article such that essentially noimprovement in mechanical strength is felt. Extensive investigation hasdemonstrated that one of the major causes of the failure of monovalentmetal salt baths to strengthen alkali metal silicate glass articles isthe presence of alkaline earth metal ions in the bath. Hence, amountsgreater than about 1 percent by weight of calcium ions in the moltensalt can exchange with an alkali metal cation in the glass surface,thereby blocking any further ion exchange. The other alkaline earthmetals, i.e., strontium and barium, are much larger and less mobile suchthat exchange therewith is substantially negligible. These calcium ionsappear in the bath of molten salt as unwanted impurities in the salt,itself, and/or as a result of the ion exchange reaction wherein calciumions from the glass surface diffuse out into the bath.

U.S. Pat. No. 3,395,999, assigned to the same assignee as the presentapplication, discloses the addition of diatomaceous earth to monovalentmetal salt baths to minimize chemical attack of the glass surface and toconvert a non-strengthening bath to a strengthening bath. That inventionrelied upon the extensive surface area offered by the diatomaceous earthfor reaction with impurities in the bath. However, although effective inreducing chemical attack by the bath on the glass surface andmaintaining the strengthening capability of the bath, threedisadvantages were observed then practicing that invention: (1) thepresence of diatomaceous earth in the bath increased the viscositythereof such as to impede easy handling of the glass and increasing thetime required for removing the salt clinging to the glass articles whenlifted out of the bath; (2) continued additions of diatomaceous earth tothe bath to maintain the strengthening capability thereof produced asalt bath mixture too viscous for convenient handling of the articles;and (3) the diatomaceous material settled to the bottom of the bath tankcausing the burning out of the heating elements placed therein.

Because of these problems, another solution for removing impurities fromthe bath was sought. I have discovered that calcium ions in a salt bathcan be tied up in such a way by passing water vapor through the moltensalt. Hence, by passing water vapor through the molten salt, the speciesCa( H O),," wherein n= 14, can be produced in the melt. lnasmuch asthese hydrated ions are very large in size, they are incapable ofexchanging with the alkali metal ions in the glass surface and,therefore, their presence in the molten salt is rendered innocuous. Yet,since the Ca(H O) species remain in solution, the three problemsencountered with diatomaceous earth recited above are absent in myinvention.

Whereas the passing of water vapor through baths of molten chloride,sulfate, and other high melting salts which are useful in practicing theinventions of U.S. Pat. No. 2,779,136 and British Pat. Nos. 917,388 and966,733 is unquestionably beneficial, the principal commercial glassstrengthening processes involving ion exchange utilize nitrate bathsoperating at temperatures below about 600 C. and, consequently, thefollowing specific examples of my invention are directed thereto. Hence,commercial glass strengthening by ion exchange of thelarge-ion-for-small-ion type has customarily employed a nitrate bathwherein large monovalent ions replace smaller alkali metal ions in thesurface of the glass at temperatures below the strain point of theglass. Therefore, the examples described below particularly refer toglasses and baths of molten salts utilized in that type of chemicalstrengthening practice.

Glass cane samples about 4 in. X A diameter to be broken for modulus ofrupture measurements were formed from a composition, expressed in weightpercent on the oxide basis, of approximately 61.4% SiO 16.8% A1 0 12.7%Na O, 3.6% K 0, 3.6% MgO, 1.0% AS203, 0.2% CaO, and 0.7% TiO The glassbatch may consist of any ingredients, either the oxides or othercompounds, which, on being melted together, will be converted to thedesired oxide composition in the proper proportions. This glass isespecially suitable for strengthening by means of a bath of moltenpotassium salt such that K ions are exchanged for Na ions.

This glass composition comes within the purview of British Pat. No.966,733 and specific reference is made thereto for other alkali metalaluminosilicate glasses which are suitable for use in this invention.

The following table reports the results of the invention. Thus, a freshbath of molten Bakers reagent grade potassium nitrate, KNO was preparedand cane samples of the above glass immersed therein at 525 C. for thetimes noted. The cans samples were then removed, from the bath, theclinging salt washed off with tap water, and modulus of rupture valuesmeasured in the conventional manner on each. The figure recorded in thetable represents an average value for five cane samples.

Thereafter, about 0.22 percent, 0.5 percent, and 1 percent by weight ofcalcium nitrate, Ca(NO were admixed to the bath of molten KNO tosimulate contamination of the bath with Ca ions leached out of the glassand/or present as impurities in the KNO constituting the bath. Canesamples were immersed into this so-contaminatedbath at 525 C. for thetimes noted. The samples were then withdrawn from the bath, washed intap water, and modulus of rupture values determined thereon.

Finally, nitrogen, passed through a flask of water at a rate to yield 1liter of water vapor per hour, was bubbled through the contaminated saltbath for the times reported. Cane samples were then immersed into thebath at 525 C. for the times noted, the samples thereafter removed fromthe bath, cleaned of salt in tap water, and modulus of rupture valuesmeasured thereon. I

1 l./hr. water vapor for 16 hours The effectiveness of passing watervapor through the bath of molten KNO in restoring the strengtheningcapability thereof which had been seriously depressed through thepresence of Ca ion contamination is believed to be dramaticallydemonstrated in this table. Furthermore, the table ably illustrates thecriticality of maintaining the contamination of the bath with Ca ionsbelow a maximum of about 1 percent by weight. Hence, even very extendedperiods of bubbling water vapor through baths containing more than about1 percent Ca ion contamination will not yield an article having amechanical strength approaching that obtainable from an essentially Caion-free bath.

Although the examples reported in the table were founded upon the use ofa highly contaminated bath, it will be appreciated that, rather thanwaiting until the bath of molten salt has lost much of its strengtheningcapability (as was simulated in the above tests) and then beginning thewater vapor treat-v ment thereof, short periods of vapor flow atempirically determined s aced intervals could be utilized or essentiagyc ontmuous ow at low water volume would likewise be e ectlve.

Also, whereas nitrogen was employed in the preceding examples as thecarrier gas because of its relative cheapness, other inert gases such ashelium and argon may be substituted therefore or Water vapor alone maybe bubbled through the melt. However, the quantity of water vaporintroduced, the ease with which the water vapor can be dispersed in thebath, and the safety factor inherent in bubbling water vapor in dilutedform into the bath rather than by itself has recommended the use of aninert carrier or diluent gas. Nevertheless, that it is the water vaporwhich is effective in tying up the Ca ions to restore the strengtheningpotential of a Ca ion contaminated bath rather than the inert diluents,has been experimentally confirmed.

Finally, the rate at which the water vapor is passed into the bath isonly important in that the quantity of vapor is adequate to restoreand/or maintain the Ca ion concentration below 1 percent. Obviously,excessive amounts merely increase production costs and excessive flowrates will cause unnecessary turbulence in the bath.

In any event, it is firmly believed that each of these process factorscan be readily determined empirically and, therefore, is well within thetechnical competence of a person of ordinary skill in the art.

I claim:

1. In a method for continuously strengthening an alkali metal silicateglass article by immersing said article in a bath of molten alkali metalsalt for a period of time, said bath being contaminated with Ca ionsleached out of the glass and/or present as impurities in the salt bath,the improvement which comprises passing water vapor through said bath ofmolten salt to reduce and/or maintain the amount of Ca ions at less thanabout 1 percent by weight of the molten salt bath.

2. In a method in accordance with claim 1 wherein said glass articleconsists essentially, by weight on the oxide basis, of about 5-25percent alkali metal oxide, 5-25 percent by weight A1 0 and theremainder SiO 3. In a method in accordance with claim 2 wherein saidalkali metal oxide is Na O.

4. In a method in accordance with claim 1 wherein an inert gas isutilized as a carrier for said water vapor.

5. In a method in accordance with claim 4 wherein said inert gas isnitrogen.

2. In a method in accordance with claim 1 wherein said glass article consists essentially, by weight on the oxide basis, of about 5-25 percent alkali metal oxide, 5-25 percent by weight Al2O3, and the remainder SiO2.
 3. In a method in accordance with claim 2 wherein said alkali metal oxide is Na2O.
 4. In a method in accordance with claim 1 wherein an inert gas is utilized as a carrier for said water vapor.
 5. In a method in accordance with claim 4 wherein said inert gas is nitrogen. 